Critical dimension measuring method

ABSTRACT

Line profile data obtained by scanning a circuit pattern with an electron beam is smoothing-differential so as to obtain a pair of approximate edge positions of the circuit pattern. Positions apart from the pair of approximate edge positions by a predetermined number of picture elements are referred to as start point and end point of automatic measuring algorithm. The automatic measuring algorithm is performed for the line profile data between the start point and the end point so as to detect precise edge positions of the circuit pattern.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method for measuring the size of acircuit pattern formed on a substrate in an LSI fabrication process, inparticular, to a circuit pattern size measuring method for accuratelydetecting edge positions of a circuit pattern.

2. Description of the Related Art

Conventionally, edge positions of circuit patterns formed on substratesin LSI fabrication process are detected by scanning electron microscopes(SEM). The SEM scans a circuit pattern with an electron beam so as todetect a circuit pattern. Thereafter, the operator manually setsparallel cursors on the circuit pattern obtained as a SEM image. Anautomatic measuring algorithm is executed for line profile data in theregion surrounded by the parallel cursors so as to detect the edgepositions of the circuit pattern.

Thus, to designate a measuring region of a circuit pattern for which theautomatic measuring algorithm is executed, the operator should manuallyset the parallel cursors. However, the setting of the parallel cursorsdeviates depending on each operator. Therefore, the setting accuracy ofthe circuit pattern required for the automatic measuring algorithmcannot be always satisfied. In addition, available automatic measuringalgorithms (such as threshold method, linear regression method, andmaximum differential method) sometimes result in deviation of measuredvalues or measurement error depending on parallel cursors being set.Moreover, the manual setting of parallel cursors lowers the operabilityand requires longer measurement time. The present invention is made tosolve such problems.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a critical dimensionmeasuring method for accurately detecting edge positions of a circuitpattern without necessity of manual setting of parallel cursors.

The present invention is a circuit pattern size measuring method,comprising the steps of scanning a circuit pattern on a substrate withan electron beam so as to form image data, obtaining line profile dataof the circuit pattern corresponding to measurement positions inaccordance with the image data, smoothing-differentiating the lineprofile data so as to obtain a smoothing-differential wave form,calculating maximal values of the line profile data in accordance withthe smoothing-differential wave form and obtaining measurement positionscorresponding to two biggest points of each of the maximal values as apair of approximate edge positions of the circuit pattern, obtaining astart point and an end point of an automatic measuring algorithmcorresponding to the pair of approximate edge positions, and performingthe automatic measuring algorithm for the line profile data ranging fromthe start point to the end point, for measuring a pair of accurate edgeportions of the circuit pattern.

According to the present invention, the line profile data issmoothing-differentiated so as to obtain a pair of approximate edgepositions of a circuit pattern. Corresponding to the pair of approximateedge positions, a start point and an end point are designated. With thestart point and the end point, an automatic algorithm is executed. Thus,edge positions of a circuit pattern can be accurately detected.

These and other objects, features and advantages of the presentinvention will become more apparent in light of the following detaileddescription of a best mode embodiment thereof, as illustrated in theaccompanying drawings.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic diagram showing an apparatus for accomplishing acritical dimension measuring method according to an embodiment of thepresent invention;

FIG. 2 is a sectional side view showing a sample where a circuit patternis formed on a substrate;

FIG. 3 is a sectional side view showing the sample where the circuitpattern on the substrate is scanned with an electron beam;

FIG. 4 is a graph showing normalized line profile data;

FIG. 5 is a graph showing a wave form where line profile data has beensmooth-differentiated;

FIG. 6 is a schematic diagram showing a start point and an end point ofan automatic measuring algorithm; and

FIGS. 7a-h are schematic diagram showing arithmetic operations forRobinson operator process.

DESCRIPTION OF PREFERRED EMBODIMENT

Next, with reference to the accompanying drawings, an embodiment of thepresent invention will be described. FIGS. 1 to 7 show a criticaldimension measuring method according to an embodiment of the presentinvention.

FIG. 1 shows an apparatus which accomplishes the circuit dimensionmeasuring method according to the present invention. The apparatusincludes a scanning electron microscope (SEM) 1 which comprises an X-Ystage 4 and a deflector 2. The X-Y stage 4 holds a sample 5. Thedeflector 2 deflects an electron beam 25 (see FIG. 3) so as to scan thesample 5 with the electron beam 25. As shown in FIG. 2, the sample 5 iscomposed of a substrate (such as a wafer) and a circuit pattern 21formed thereon. A detector 3 is disposed above the X-Y stage 4. Thedetector 3 detects secondary electrons from the sample 5. The detector 3is connected to an image processing device 7. The detector 3 outputs asecondary electron signal to the image processing device 7. The imageprocessing device 7 outputs a deflection signal to the deflector 2. Theimage processing device 7 is connected to a computer 6 and a monitor 8.

Next, the operation of this embodiment will be described. The imageprocessing device 7 outputs the deflection signal to the deflector 2 ofthe SEM 1. The deflector 2 scans the circuit pattern 21 formed on thesubstrate 22 with the electron beam 25 (see FIG. 3). At this point, thedetector 3 of the SEM 1 detects secondary electrons from the sample 5.The detector 3 outputs the secondary electron signal to the imageprocessing device 7 in synchronization with the deflection signal. Theimage processing device 7 converts the secondary electron signal, whichis an analog signal, into a digital signal. The digital data which hasgraduation of 256 levels is stored in a frame memory 7a. In thisembodiment, the scanning operation with the electron beam 25 may beperformed several times so as to reduce noise. The obtained image datamay be averaged and then stored in the frame memory 7a.

The computer 6 reads the image data from the frame memory 7a andperforms arithmetic operations for measuring the size of a circuitpattern. These arithmetic operations will be described later. Inaddition, a circuit pattern corresponding to the image data stored inthe frame memory 7a of the image processing device 7 is displayed on amonitor 8.

Next, the arithmetic operations for measuring the size of a circuitpattern performed by the computer 6 will be described. First, image datais read from the frame memory 7a. The computer 6 calculates line profiledata corresponding to measuring positions in beam scanning direction (xdirection). Then, the computer 6 calculates the maximum value and theminimum value of the line profile data and then normalizes the lineprofile data. The normalizing operation is performed in such a way thatthe line profile data is linearly emphasized so that the minimum valueand the maximum value become 0 and 255, respectively. FIG. 4 showsnormalized profile data F (X_(N)).

Next, the normalized line profile data F (X_(N)) issmooth-differentiated so as to detect the edges of the circuit pattern21. The smoothing-differential method is a process which convolutesstandardized line profile data F (X_(N)) with weighting coefficients forthe smooth-differentiation (e.g., -3, -2, -1, 0, 1, 2, 3 in the case ofseven-point smoothing of the first- order differentiation) so as toobtain a smoothing-differential wave form.

Thus, the normalized line profile data F (X_(N)) is convoluted with thesmoothing-differential weighting coefficients so as to obtain asmoothing-differential wave form F'(X_(N)) (see FIG. 5).

Next, measurement positions X_(K) where the smoothing-differential waveform F'(X_(N)) changes from positive to negative are obtained. X_(K) areequivalent to measurement positions where the line profile data F(X_(N)) has maximal values. To detect two peaks according to the edgesof the circuit pattern 21, measurement positions X₁ and X₂ of X_(K)where F (X_(K))>A are obtained. In other words, X₁ and X₂ are themeasurement positions corresponding to two biggest points of the maximalvalues. Next, the value of X₁ is compared with the value of X₂. Thesmaller value is referred to as an approximate left edge positionX_(MAXL), whereas the larger value is referred to as an approximateright edge position X_(MAXR).

In addition, the number of predetermined picture elements is referred toas B. The position where the approximate left edge position X_(MAXL) ismoved leftward (outward) by B, (X_(MAXL) -B), is referred to as a leftparallel cursor position C_(L) (start point), whereas the position wherethe approximate right edge position X_(MAXR) is moved rightward (inward)by B, (X_(MAXR) +B), is referred to as a right parallel cursor positionC_(R) (end point). The parallel cursor positions C_(L) and C_(R) areused for the automatic measuring algorithm which will be describedlater. In FIG. 6, the approximate edge positions X_(MAXL) and X_(MAXR)accord with slope portions 32 of the circuit pattern 31 of the imagedata.

Thus, with the automatic measuring algorithm, the start point and theend point of the line profile data F (X_(N)) to be measured areautomatically specified. Next, for the region from the start point C_(L)to the end point C_(R) of the line profile data F (X_(N)), the automaticmeasuring algorithm is performed. Examples of the automatic measuringalgorithm are threshold method, liner regression method, and maximumdifferential method. By the automatic measuring algorithm, the edgepositions of the circuit pattern 21 can be precisely obtained.

According to this embodiment, the line profile data issmooth-differentiated so as to obtain the approximate edge positionsX_(MAXL) and X_(MAXR). Corresponding to these approximate edgepositions, the left and right parallel cursor positions C_(L) and C_(R)are set. Thus, the parallel cursors can be more accurately set than theconventional method where they are manually set. Thus, by the automaticmeasuring algorithm, the edge positions of a circuit pattern can beaccurately detected.

In the above-described embodiment, the image data with gradation of 256levels was stored in the frame memory 7a. However, it should be notedthat the present invention is not limited to such gradation. Instead,image data with gradation of 4096 may be stored in the frame memory 7a.

In the above-described embodiment, the line profile data in the Xdirection was obtained. However, before obtaining the line profile data,Robinson operator process as shown in FIG. 7 may be performed so as toobtain line profile data in any desired direction.

For example, if the Robinson operator processing shown in FIG. 7(a) isperformed, one edge of the circuit pattern which runs in parallel to thea X direction of the image data stored in the frame memory 7a isemphasized. In addition, by the Robinson operator processing shown inFIG. 7(b), one edge of the circuit pattern which runs diagonally fromthe upper right to the lower left is emphasized. By the Robinsonoperator processing shown in FIG. 7(c), one edge of the circuit patternwhich runs vertically is emphasized. By the Robinson operator processingshown in FIG. 7(d), one edge of the circuit pattern which runs from theupper left to the lower right is emphasized.

By the Robinson operator processing shown in FIG. 7(e), the other edgeof the circuit pattern in a same direction as FIG. 7(a) is emphasized.By the Robinson operation process shown in FIG. 7(f), the other edge ofthe circuit pattern in a same direction as FIG. 7(b) is emphasized. Bythe Robinson operator processing shown in FIG. 7(g), the other edge ofthe circuit pattern in a same direction as FIG. 7(c) is emphasized. Bythe Robinson operator processing shown in FIG. 7(h), the other end ofthe circuit pattern in a same direction as FIG. 7(d) is emphasized.

By successively performing the Robinson operator process in theabove-described manners, image data which is emphasized in eachdirection is compared. Thus, the direction of the circuit pattern isdetected. Corresponding to the detected direction, the line profile datacan be obtained.

According to the present invention, the start point and the end point ofthe automatic algorithm can be specified corresponding to the pair ofapproximate edge positions obtained by smoothing and differentiation ofthe line profile data. Thus, the edge positions of the circuit patterncan be more accurately detected than the conventional method whereparallel cursors are manually set.

Although the present invention has been shown and described with respectto a best mode embodiment thereof, it should be understood by thoseskilled in the art that the foregoing and various other changes,omissions, and additions in the form and detail thereof may be madetherein without departing from the spirit and scope of the presentinvention.

What is claimed is:
 1. A critical dimension size measuring methodcomprising the steps of:(a) scanning a circuit pattern on a substratewith an electron beam so as to form image data; (b) obtaining lineprofile data of the circuit pattern corresponding to measurementpositions in accordance with said image data; (c) smooth-differentiatingsaid line profile data so as to obtain a smoothing differentialwaveform; (d) calculating maximal values of the line profile data inaccordance with the smoothing differential waveform and obtainingmeasurement positions corresponding to the maximal values as a pair ofapproximate edge positions of the circuit pattern; (e) obtaining a startpoint and an end point of an automatic measuring algorithm correspondingto the pair of approximate edge positions; and (f) performing theautomatic measuring algorithm for the line profile data ranging from thestart point to the end point, for measuring a pair of actual edgepositions of the circuit pattern.
 2. The critical dimension sizemeasuring method as set forth in claim 1,wherein said step (b) isperformed by averaging image data obtained by scanning the circuitpattern with the electron beam a plurality of times.
 3. The criticaldimension size measuring method as set forth in claim 1, furthercomprising the steps of:(b-1) detecting a maximum value of and a minimumvalue of the line profile data; and (b-2) linearly emphasizing the lineprofile data in accordance with the maximum value and the minimum valueso as to standardize the line profile data.
 4. The critical dimensionsize measuring method as set forth in claim 1,wherein said step (e) isperformed by designating a position spaced apart leftward from anapproximate left edge position by a predetermined number of pictureelements as a start point and a position spaced apart rightward from anapproximate right edge position by a predetermined number of pictureelements as an end point.
 5. A critical dimension size measuring method,comprising the steps of:(a) scanning a circuit pattern on a substratewith an electron beam so as to form image data; (b) obtaining lineprofile data of the circuit pattern corresponding to measurementpositions in accordance with said image data; (c) smooth-differentiatingthe line profile data so as to obtain a smoothing differential waveform;(d) calculating maximal values of the line profile data in accordancewith the smoothing differential waveform and obtaining measurementpositions correspond to the maximal values as a pair of approximate edgepositions of the circuit pattern; (e) obtaining a start point and an endpoint of an automatic measuring algorithm corresponding to the pair ofapproximate edge positions; and (f) performing the automatic measuringalgorithm for the line profile data ranging from the start point to theend point, for measuring a pair of actual edge positions of the circuitpattern, wherein said step (b) is performed by executing a Robinsonoperator process for the image data, said Robinson operator process forthe image data, said Robinson operator process being adapted toemphasize the approximate edge positions of the circuit pattern in adesirable direction.
 6. The critical dimension size measuring method asset forth in claim 5,wherein said step (b) is performed by averagingimage data obtained by scanning the circuit pattern with the electronbeam a plurality of times.
 7. The critical dimension size measuringmethod as set forth in claim 5, further comprising the steps of:(b-1)detecting a maximum value and a minimum value of the line profile data;and (b-2) linearly emphasizing the line profile data in accordance withthe maximum value and the minimum value so as to standardize the lineprofile data.
 8. The critical dimension size measuring method as setforth in claim 5,wherein said step (e) is performed by designating aposition spaced apart leftward from an approximate left edge position bya predetermined number of picture elements as a start point and aposition spaced apart rightward from an approximate right edge positionby a predetermined number of picture elements as an end point.